Page 213 - Fluid mechanics, heat transfer, and mass transfer

MIXING EQUIPMENT 191

from this segregated pile occurs from the central ➢ For example, many pharmaceutical dry blends
core, then a concentration of fine particles will must be highly free flowing to allow flow through
occur, eventually followed by the coarse small orifices in a tableting press or encapsulation
material. equipment. However, this feature commonly al-
– This mechanism requires four conditions, lows segregation of the formulation.
namely, difference in particle sizes, relatively ➢ Product formulators may be capable of altering the
large particles (average size greater than cohesiveness of the mix using liquid binders or
100 mm), free-flowing material, and interparti- changing particle size distribution, although a
cle motion. significant change could induce other flow pro-
– If any one of the four is absent, the mixture will blems, such as bridging or rat holing, which
not segregate by this mechanism. immediately creates a process bottleneck.
➢ Fluidization Segregation: This type of segrega- & Changing the Process or Equipment: The following
tion results when finer, lighter particles rise to the general practices can help to prevent or minimize the
top surface of a fluidized mix of powder, while the effects of segregation:
larger, heavier particles concentrate at the bottom ➢ Mixing process should be located as far down-
of the bed. In this case, the fluidizing air entrains stream in the process as possible.
the lower permeability fines and carries them to the ➢ Post-blend handling of the material should be
top surface. minimized.
– This mechanism generally occurs only with ➢ Surge and storage bins should be designed for
powders with an average particle size smaller mass flow (no stagnant regions in hopper).
than 100 mm. ➢ Velocity gradients within bins should be
– Fluidization segregation is likely to occur when minimized.
fine materials are pneumatically conveyed, ➢ Asymmetric hoppers should be avoided due to
when they are filled or discharged at high rates,
their large velocity gradients.
or if gas counterflow occurs. As with most
➢ A mass flow bin with a tall, narrow cylinder will
segregation mechanisms, the more cohesive the
minimize the potential for segregation compared
material, the less likely it will segregate by this
to that of a short, wide bin. It is preferred to keep
mechanism.
the material level in the bin high.
➢ Dusting Segregation: Finally, this form of segre-
➢ Design symmetry should be utilized whenever
gation concentrates the ultrafine and fine particles
possible. For example, if designing a bin with
at container’s walls or at points farthest from the
multiple outlets, all of the outlets should be located
incoming stream of material.
at the same distance from the bin centerline.
– Dusting segregation is a common problem with
➢ Ensuring that any splitting of a process stream
fine pharmaceutical and food powders being
does not result in differences between the streams.
discharged from mixers into drums, tableting
➢ Venting should be used to avoid air counterflow
press hoppers, and packaging equipment surge
(which induces fluidization segregation).
hoppers. This effect starts to become prevalent
around 50 mm and is very common below ➢ Generation of dust should be minimized.
10 mm. . What are the parameters involved in solids mixing?
& By knowing which segregation mechanisms, if any, & It is not possible to achieve a completely uniform
might dominate, precautions can be taken in the mixture of dry powders or particulate solids. The
design and equipment selection process to avert degree of mixing achieved depends on the
problems. following:
➢ The relative particle size, shape, and density.
. How segregation effects are reduced?
➢ The efficiency of the particular mixer for the
& There are three general approaches for the reduction
components being mixed.
of segregation problems, namely, changing the ma-
terial, changing the process, or changing the equip- ➢ The tendency of the materials to aggregate.
ment design. ➢ The moisture content, surface characteristics, and
& Changing the Material: Highly segregating mate- flow characteristics of each component.
rials are free flowing, and thus prone to sifting & Generally, materials similar in size, shape, and den-
segregation. sity are able to form the most uniform mixtures.